conjunctival injection

Conjunctival injection results from vasodilation of the small superficial blood vessels within the bulbar and/or palpebral conjunctiva, typically in response to inflammation, irritation, infection, allergy, or increased blood flow. On exam, the vessels appear bright red and are movable with the conjunctiva (distinguishing it from ciliary/episcleral injection, which is fixed and indicates deeper, more serious pathology like uveitis, scleritis, or acute angle-closure glaucoma).

Common causes include: - Viral conjunctivitis (most common cause of acute red eye, often with watery discharge, preauricular lymphadenopathy, adenovirus) - Bacterial conjunctivitis (purulent discharge) - Allergic conjunctivitis (itching, bilateral, cobblestone papillae) - Dry eye syndrome - Subconjunctival hemorrhage (localized, not diffuse injection) - Systemic diseases: Kawasaki disease (bilateral nonexudative conjunctival injection is a key diagnostic criterion), Stevens-Johnson syndrome, measles, leptospirosis - Contact lens overwear or foreign body - Pterygium/pinguecula

Clinical significance: Differentiating conjunctival injection from ciliary flush is critical in the 'red eye' differential—ciliary flush (perilimbal injection) suggests more vision-threatening diagnoses (uveitis, keratitis, acute angle-closure glaucoma) requiring urgent ophthalmologic referral, whereas conjunctival injection alone is more often benign (conjunctivitis). Associated symptoms (pain, photophobia, vision changes, pupil abnormalities) help risk-stratify the red eye presentation.

On USMLE, conjunctival injection is a classic finding tested in the context of Kawasaki disease diagnostic criteria (fever + conjunctivitis + rash + adenopathy + mucositis + extremity changes), viral vs bacterial vs allergic conjunctivitis distinction, and the broader 'red eye' differential diagnosis requiring identification of emergent vs non-emergent causes.